Semiconductor device with dummy gate and metal gate and method of fabricating the same

ABSTRACT

A semiconductor device includes a substrate, a first well, a second well, a metal gate, a poly gate, a source region, and a drain region. The first well and the second well are within the substrate. The metal gate is partially over the first well. The poly gate is over the second well. The poly gate is separated from the metal gate, and a width ratio of the poly gate to the metal gate is in a range from about 0.1 to about 0.2. The source region and the drain region are respectively within the first well and the second well.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims priority to U.S. Provisional Application Ser. No. 62/865,833, filed Jun. 24, 2019, which is herein incorporated by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in integrated circuit materials and design have produced generations of integrated circuits where each generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing integrated circuits and, for these advances to be realized, similar developments in integrated circuit processing and manufacturing are needed. In the course of integrated circuit evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component that can be created using a fabrication process) has decreased.

The decreased geometry size leads to challenges in fabricating a type of transistor device known as a laterally diffused metal-oxide-semiconductor (LDMOS) transistor. The high blocking voltage ability of the LDMOS transistor can be achieved through a formation of a resistive path, which serves as a voltage drop in the channel region of the LDMOS transistor. Existing technologies use lightly doped source and drain regions to define the resistive path. As such, the resistive path is very shallow, particularly as the geometry sizes continue to shrink. The shallow resistive path may not offer resistance as high as desired for the LDMOS transistor. Further, the shrinking geometry sizes present challenges for accurate alignment and overlay control in fabricating the LDMOS transistor. Therefore, while existing methods of fabricating LDMOS transistors have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a cross-sectional view illustrating a semiconductor device in accordance with some embodiments of the present disclosure;

FIG. 2 is a flow chart illustrating a method of fabricating the semiconductor device shown in FIG. 1 in accordance with some embodiments of the present disclosure;

FIGS. 3-12 are cross-sectional views illustrating a method for fabricating the semiconductor device shown in FIG. 1 at various stages in accordance with some embodiments of the present disclosure; and

FIG. 13 is a cross-sectional view illustrating a semiconductor device in accordance with another embodiment of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

FIG. 1 is a cross-sectional view illustrating a semiconductor device 100 in accordance with some embodiments of the present disclosure. The semiconductor device 100 includes a pair of laterally diffused metal-oxide-semiconductor (LDMOS) transistors, in which the two identical LDMOS transistors in the semiconductor device 100 are symmetrical with respect to a shared drain region 150, in a cross-sectional view shown in FIG. 1. In other words, the shared drain region 150 serves as a symmetrical centre of the semiconductor device 100 in a cross-sectional view shown in FIG. 1. The semiconductor device 100 includes a substrate 110, two first wells 120 a and 120 b, a second well 130, two source regions 140 a and 140 b, a drain region 150, two metal gates 160 a and 160 b, and two poly gates 170 a and 170 b. The first wells 120 a and 120 b and the second well 130 are within the substrate 110, in which the second well 130 is between the first wells 120 a and 120 b, and the first wells 120 a and 120 b and the second well 130 are separated by portions of the substrate 110. The source regions 140 a and 140 b are respectively within the first wells 120 a and 120 b, and the drain region 150 is within the second well 130 and between the two poly gates 170 a and 170 b to be shared by the two LDMOS transistors. The metal gate 160 a is partially over the first well 120 a and partially over the second well 130, and the metal gate 160 b is partially over the first well 120 b and partially over the second well 130. The poly gates 170 a and 170 b are over the second well 130, and are separated from each other. Additionally, the poly gates 170 a and 170 b are separated from the metal gates 160 a and 160 b.

FIG. 2 is a flow chart illustrating a method of fabricating the semiconductor device 100 shown in FIG. 1 in accordance with some embodiments of the present disclosure. The method begins with block S10 in which a first well and a second well are formed in a substrate, wherein the first well and the second well are doped with different types of dopants. The method continues with block S12 in which a dummy gate is formed partially over the first well and partially over the second well, wherein the dummy gate includes a dummy gate stack. The method continues with block S14 in which a poly gate is formed over the second well, wherein the poly gate includes a poly gate stack, and a gap is between the dummy gate and the poly gate. The method continues with block S16 in which a source region and a drain region are respectively formed in the first well and the second well, wherein the source region and the drain region are doped with a same type of dopants. The method continues with block S18 in which the dummy gate stack is replaced with a metal gate stack.

FIGS. 3-12 are cross-sectional views illustrating a method for fabricating the semiconductor device 100 shown in FIG. 1 at various stages according to some embodiments. Accordingly, it should be noted that additional processes may be provided before, during, and after the method of FIG. 2, and that some other processes may only be briefly described herein.

In the present embodiment, the semiconductor device 100 includes n-type LDMOS transistors. It is understood that a p-type LDMOS semiconductor device may be formed in an alternative embodiment. Reference is made to FIG. 3. The semiconductor device 100 includes the substrate 110. In some embodiments, the substrate 110 is a silicon substrate doped with a p-type dopant such as boron. In other embodiments, the substrate 110 is a silicon substrate doped with an n-type dopant such as arsenic or phosphorous.

Isolation structures 180 a and 180 b are formed in the substrate 110. In some embodiments, each of the isolation structures 180 a and 180 b is a shallow trench isolation (STI) structure including a dielectric material, which may be silicon oxide or silicon nitride. The first wells 120 a and 120 b and the second well 130 are formed in the substrate 110. In some embodiments, each of the first wells 120 a and 120 b is a p-well doped with a p-type dopant such as boron, and the second well 130 is an n-well doped with an n-type dopant such as arsenic or phosphorous. In some embodiments, the first wells 120 a and 120 b and the second well 130 are separated by a portion of the substrate 110.

Dummy gate stacks 192 a and 192 b and poly gate stacks 172 a and 172 b are then formed over the substrate 110. The dummy gate stack 192 a is partially formed over the first well 120 a and the second well 130, and the dummy gate stack 192 b is partially formed over the first well 120 b and the second well 130. Furthermore, the poly gate stacks 172 a and 172 b are formed over the second well 130. Additionally, gaps 50 a and 50 b are respectively between the dummy gate stack 192 a and the poly gate stack 172 a and between the dummy gate stack 192 b and the poly gate stack 172 b. In some embodiments, the dummy gate stacks 192 a and 192 b and the poly gate stacks 172 a and 172 b are formed simultaneously in a same process and include a same material. For example, the dummy gate stacks 192 a and 192 b and the poly gate stacks 172 a and 172 b respectively include gate dielectric layers 194 a and 194 b and gate dielectric layers 174 a and 174 b. In some embodiments, a material of the gate dielectric layers 194 a, 194 b, 174 a, and 174 b includes silicon oxide. In another embodiment, the gate dielectric layers 194 a, 194 b, 174 a, and 174 b include a high-k dielectric material. The high-k dielectric material is a material having a dielectric constant that is greater than a dielectric constant of SiO₂ which is approximately 4. For example, the high-k dielectric material may include hafnium oxide (HfO₂), which has a dielectric constant in a range from about 18 to about 40. Alternatively, the high-k dielectric material may include ZrO₂, Y₂O₃, La₂O₅, Gd₂O₅, TiO₂, Ta₂O₅, HfErO, HfLaO, HfYO, HfGdO, HfAlO, HfZrO, HfTiO, HfTaO, SrTiO, or combinations thereof.

The dummy gate stacks 192 a and 192 b and the poly gate stacks 172 a and 172 b further respectively include gate electrode layers 196 a and 196 b and gate electrode layers 176 a and 176 b that are respectively disposed over the gate dielectric layers 194 a and 194 b and the gate dielectric layers 174 a and 174 b. A material of the gate electrode layers 196 a, 196 b, 176 a, and 176 b includes polysilicon. The dummy gate stacks 192 a and 192 b and poly gate stacks 172 a and 172 b further respectively include hard mask layers 198 a and 198 b and hard mask layers 178 a and 178 b that are respectively disposed over the gate electrode layers 196 a and 196 b and the gate electrode layers 176 a and 176 b. The hard mask layers 198 a, 198 b, 178 a, and 178 b include a dielectric material, such as silicon oxide or silicon nitride. Although not illustrated herein, the hard mask layers 198 a, 198 b, 178 a, and 178 b were formed by patterning a hard mask material with a patterned photoresist layer. The hard mask layers 198 a, 198 b, 178 a, and 178 b were then used to pattern the gate electrode layers 196 a, 196 b, 176 a, and 176 b and the gate dielectric layers 194 a, 194 b, 174 a, and 174 b below so as to form the dummy gate stacks 192 a and 192 b and the poly gate stacks 172 a and 172 b.

Reference is made to FIG. 4. First spacers 199 a and 199 b are respectively formed on sidewalls of the dummy gate stacks 192 a and 192 b, and second spacers 179 a and 179 b are respectively formed on sidewalls of the poly gate stacks 172 a and 172 b. As a result, the gaps 50 a and 50 b reappear with smaller widths W1 and W2, respectively. The first spacers 199 a and 199 b and the dummy gate stacks 179 a and 179 b are together referred to as the dummy gates 190 a and 190 b, and the second spacers 179 a and 179 b and the poly gate stacks 172 a and 172 b are together referred to as the poly gates 170 a and 170 b. In some embodiments, the first spacers 199 a and 199 b and the second spacers 179 a and 199 b are formed simultaneously in a same process and may include the same material. For example, the first spacers 199 a and 199 b and the second spacers 179 a and 179 b are formed using a deposition process and an etching process (for example, an anisotropic etching process). The first spacers 199 a and 199 b and the second spacers 179 a and 179 b may include a suitable dielectric material such as silicon nitride, silicon oxide, silicon carbide, silicon oxy-nitride, or combinations thereof.

Reference is made to FIG. 5. Photoresist masks 200 a and 200 b are formed over the substrate 110. The photoresist masks 200 a and 200 b are formed by forming a photoresist layer over the substrate 110 and patterning the photoresist layer into the photoresist masks 200 a and 200 b in a lithography process. The photoresist mask 200 a covers a portion of the dummy gate 190 a and a portion of the poly gate 170 a and fills the gap 50 a (see FIG. 4), and the photoresist mask 200 b covers a portion of the dummy gate 190 b and a portion of the poly gate 170 b and fills the gap 50 b (see FIG. 4). The purpose of forming the photoresist masks 200 a and 200 b is to protect regions of the second well 130 underneath the gaps 50 a and 50 b (see FIG. 4) from being doped by dopants in a later implantation (or doping) process.

The source regions 140 a and 140 b are then respectively formed in the first wells 120 a and 120 b, and the drain region 150 is formed in the second well 130. The source regions 140 a and 140 b and the drain region 150 may be formed by an ion implantation process or a diffusion process. The source regions 140 a and 140 b and the drain region 150 may also be referred to as active regions. The source regions 140 a and 140 b and the drain region 150 are each doped with an n-type dopant such as arsenic or phosphorous. Since the dopants cannot penetrate through the dummy gate stacks 192 a and 192 b and the first spacers 199 a and 199 b around the dummy gate stacks 192 a and 192 b, the source region 140 a is formed to be substantially self-aligned with one of the first spacers 199 a of the dummy gate 190 a, and the source region 140 b is formed to be substantially self-aligned with one of the first spacers 199 b of the dummy gate 190 b. Similarly, since the dopants cannot penetrate through the poly gate stacks 172 a and 172 b and the second spacers 179 a and 179 b around the poly gate stacks 172 a and 172 b, the drain region 150 is formed to be substantially self-aligned with one of the second spacer 179 a of the poly gate 170 a and one of the second spacer 179 b of the poly gate 170 b. As discussed above, the photoresist masks 200 a and 200 b protects the regions of the second well 130 below the gaps 50 a and 50 b (see FIG. 4) from being implanted in this ion implantation process. After the implantation process, the dummy gate 190 a and the drain region 150 are on opposite sides of the poly gate 170 a, and the dummy gate 190 b and the drain region 150 are on opposite sides of the poly gate 170 b. In addition, the poly gate 170 a and the source region 140 a are on opposite sides of the dummy gate 190 a, and the poly gate 170 b and the source region 140 b are on opposite sides of the dummy gate 190 b.

Reference is made to FIG. 6. The photoresist masks 200 a and 200 b (see FIG. 5) are then removed by a stripping or ashing process. Thereafter, a patterned resist protection oxide (RPO) layers 210 a and 210 b are respectively formed to partially fill the gaps 50 a and 50 b (see FIG. 4). The patterned RPO layers 210 a and 210 b are formed by conformally depositing a layer of oxide material over the substrate 110, the dummy gates 190 a and 190 b and the poly gates 170 a and 170 b, and patterning the layer of oxide material with a patterned photoresist (not illustrated) in a lithography process. After being patterned, the patterned RPO layer 210 a is formed to cover a portion of the substrate 110 between the dummy gate 190 a and the poly gate 170 a, and extends to a sidewall of one of the first spacers 199 a and a sidewall of one of the second spacers 179 a between the dummy gate stack 192 a and the poly gate stack 172 a. Similarly, the patterned RPO layer 210 b is formed to cover a portion of the substrate 110 between the dummy gate 190 b and the poly gate 170 b, and extends to a sidewall of one of the first spacers 199 b and a sidewall of one of the second spacers 179 b between the dummy gate stack 192 b and the poly gate stack 172 b. The patterned RPO layer 210 a further covers a portion of the dummy gate stack 192 a and a portion of the poly gate stack 172 a, and the patterned RPO layer 210 b further covers a portion of the dummy gate stack 192 b and a portion of the poly gate stack 172 b. A silicidation process is then performed on exposed surfaces of the substrate 110 to form substantially self-aligned silicides (also referred to as salicides) 122 a and 132. That is, the patterned RPO layers 210 a and 210 b serve as silicidation masks in the silicidation process.

Referring is made to FIG. 7. An etch stop layer 220 is then conformally formed to cover the substrate 110, the dummy gates 190 a and 190 b, the poly gates 170 a and 170 b, and the patterned RPO layers 210 a and 210 b. In an alternative embodiment, the patterned RPO layers 210 a and 210 b are removed before the etch stop layer 220 is formed, such that the etch stop layer 220 is directly in contact with the first spacer 199 a, the second spacer 179 a, and the substrate 110 which are between the dummy gate stack 192 a and the poly gate stack 172 a. Similarly, in such an embodiment, the etch stop layer 220 is directly in contact with the first spacer 199 b, the second spacer 179 b, and the substrate 110 which are between the dummy gate stack 192 b and the poly gate stack 172 b. Thereafter, an interlayer dielectric (ILD) layer 230 is formed over the substrate 110 to cover the etch stop layer 220. The ILD layer 230 may be formed by chemical vapor deposition (CVD), high density plasma chemical vapor deposition (HDP CVD), spin-on, sputtering, or other suitable methods. In some embodiments, a material of the ILD layer 230 includes silicon oxide. In other embodiments, the ILD layer 230 may include silicon oxy-nitride, silicon nitride, or a low-k material.

Reference is made to FIG. 8. A chemical-mechanical polishing (CMP) process is performed, such that the dummy gates 190 a and 190 b, the poly gates 170 a and 170 b, top portions of the patterned RPO layers 210 a and 210 b, and top portions of the etch stop layer 220 are exposed. Following by the CMP process, top surfaces 191 a and 191 b of the of the dummy gates 190 a and 190 b and top surfaces 171 a and 171 b of the poly gates 170 a and 170 b are substantially coplanar with a top surface 231 of the ILD layer 230 on either side of the dummy gates 190 a and 190 b and either side of the poly gates 170 a and 170 b. Furthermore, the etch stop layer 220 is divided into several portions after the CMP process. For example, the etch stop layer 220 is divided into three portions, in which a first portion 220′ of the etch stop layer 220 directly covers the first wells 120 a and 120 b of the substrate 110 and a portion of the dummy gates 190 a and 190 b, a second portion 220″ of the etch stop layer 220 is over the second well 130 of the substrate 110 and directly covers the patterned RPO layers 210 a and 210 b, and the third portion 220′″ of the etch stop layer 220 directly covers the drain region 150 within second well 130 of the substrate 110 and portions of the poly gates 170 a and 170 b. Since the patterned RPO layers 210 a and 210 b has not been removed, the second portion 220″ of the etch stop layer 220 is substantially higher than the first portion 220′ and the third portion 220′″ of the etch stop layer 220. Although not illustrated, one or more annealing processes are performed on the semiconductor device 100 to activate the source regions 140 a and 140 b and the drain regions 150 before or after the CMP process.

Reference is made to FIG. 9. A hard mask 240 is formed over the substrate 110 to cover the poly gates 170 a and 170 b. The purpose of forming the hard mask 240 is to protect the poly gate stacks 172 a and 172 b of the poly gates 170 a and 170 b from being removed in a later etching process. In some embodiments, a width W3 of the hard mask 240 is in a range from about 0.4 μm to 3.0 μm, which is larger than two times of a width W4 of each of the poly gates 170 a and 170 b that is in a range from about 0.02 μm to 0.2 μm. Such a width W3 of the hard mask 240 can ensure the poly gates 170 a and 170 b (or at least the poly gate stacks 172 a and 172 b of the poly gates 170 a and 170 b) being protected under the hard mask 240. For example, if the width W3 of the hard mask 240 is smaller than about 0.4 μm, the poly gates 170 a and 170 b may possibly be exposed to be under a risk of being removed in the later etching process. In some embodiments, the hard mask 240 is a photoresist mask which is formed by forming a photoresist layer over the substrate 110 and patterning the photoresist layer into the hard mask 240 in a lithography process. In some embodiments, a material of the hard mask 240 may include polymer. For example, the hard mask 240 may include a polymer material with t-butyloxycarbonyl (t-BOC) functional groups. In other embodiments, a material of the hard mask 240 may include metal.

Reference is made to FIG. 10. The dummy gate stacks 192 a and 192 b (see FIG. 9) are then removed, thereby forming trenches 250 a and 250 b respectively in places of the dummy gate stacks 192 a and 192 b. The dummy gate stacks 192 a and 192 b may be removed in a wet etching or a dry etching process, while the rest of the layers of the semiconductor device 100 including the first spacers 199 a and 199 b, the ILD layer 230, the etch stop layer 220, and the patterned RPO layers 210 a and 210 b remain substantially unetched. Additionally, since the poly gates 170 a and 170 b are protected by the hard mask 240, the poly gates 170 a and 170 b including the poly gate stacks 172 a and 172 b and the second spacers 179 a and 179 b are also remained substantially unetched. In an alternative embodiment which the gate dielectric layers 194 a and 194 b (see FIG. 3) include a high-k dielectric material (instead of silicon oxide) as discussed above, the gate dielectric layers 194 a and 194 b are not removed. This alternative embodiment will be discussed in FIG. 13.

Reference is made to FIG. 11. A material of a high-k gate dielectric layer 162 is conformally formed in the trenches 250 a and 250 b (see FIG. 10) and covers the substrate 110, the ILD layer 230, the etch stop layer 220, the patterned RPO layers 210 a and 210 b, the first spacers 199 a and 199 b, and the hard mask 240. The material of a high-k gate dielectric layer 162 may be formed by CVD, physical vapor deposition (PVD), or other suitable techniques. The material of the high-k gate dielectric layer may include the high-k dielectric material as discussed above. Although not illustrated, it is understood that a material of an interfacial layer may be formed before the material of the high-k gate dielectric layer 162 is formed. Thereafter, a material of a first conductive layer 164 is formed over the material of the high-k gate dielectric layer 162. The material of the high-k gate dielectric layer 162 may include an n-type work function metal (n-metal), which may be titanium (Ti), aluminum (Al), tantalum (Ta), ZrSi₂, TaN, or combinations thereof. Each of the n-metals has a respective range of work functions values associated therein. The material of the first conductive layer 164 may be formed by CVD, PVD, or other suitable techniques. After that, a material of a second conductive layer 166 is formed over the material of the first conductive layer 164. The material of the second conductive layer 166 may include one of tungsten (W), Aluminum (Al), copper (Cu), and combinations thereof. The second conductive layer 166 may be formed by CVD, PVD, plating, or other suitable techniques.

Reference is made to FIG. 12. A CMP process is performed, such that the ILD layer 230, the etch stop layer 220, the patterned RPO layers 210 a and 210 b, and the poly gates 170 a and 170 b are exposed. In the embodiment which the material of the hard mask 240 includes metal, the hard mask 240 is removed along with top portions of the material of the high-k gate dielectric layer 162 (see FIG. 11), the material of the first conductive layer 164 (see FIG. 11), and the material of the second conductive layer 166 (see FIG. 11) during the CMP process. In the alternative embodiment which the material of the hard mask 240 includes polymer, the CMP process is first performed to remove top portions of the material of the high-k gate dielectric layer 162 (see FIG. 11), the material of the first conductive layer 164 (see FIG. 11), and the material of the second conductive layer 166 (see FIG. 11) to expose the hard mask 240 (see FIG. 11). Thereafter, the hard mask 240 is removed by a stripping or ashing process. After that, the CMP process is continuously performed to remove the remaining layers, such that the ILD layer 230, the etch stop layer 220, the patterned RPO layers 210 a and 210 b, and the poly gates 170 a and 170 b are exposed. As a result, the metal gate stack 162 a including the high-k gate dielectric layer 164 a, the first conductive layer 166 a, and the second conductive layer 168 a is formed in the trench 250 a (see FIG. 10) and between the first spacers 199 a, and the metal gate stack 162 b including the high-k gate dielectric layer 164 b, the first conductive layer 166 b, and the second conductive layer 168 b is formed in the trench 250 b (see FIG. 10) and between the first spacers 199 b. The metal gate stack 162 a and the first spacers 199 a are together referred to as the metal gate 160 a, and the metal gate stack 162 b and the first spacers 199 b are together referred to as the metal gate 160 b. In addition, top surfaces 161 a and 161 b of the metal gates 160 a and 160 b are substantially coplanar with the top surfaces 171 a and 171 b of the poly gates 170 a and 170 b. In other words, top surfaces 161 a and 161 b of the second conductive layers 168 a and 168 b of the metal gates 160 a and 160 b are coplanar with top surfaces 171 a and 171 b of the hard mask layers 178 a and 178 b of the poly gates 170 a and 170 b.

In some embodiments, the first conductive layers 166 a and the second conductive layer 168 a together constitute the gate electrode portion of the metal gate stack 162 a, and the first conductive layers 166 b and the second conductive layer 168 b together constitute the gate electrode portion of the metal gate stack 162 b. The first conductive layers 166 a and 166 b tune a work function of the semiconductor device 100 such that a desired threshold voltage is achieved. Thus, the first conductive layers 166 a and 166 b may also be referred to as work function metal layers. Additionally, the second conductive layers 168 a and 168 b respectively serve as the main conductive portions of the metal gate stacks 162 a and 162 b and may be referred to as fill metal layers.

Since the semiconductor device 100 includes the metal gates 160 a and 160 b and the poly gates 170 a and 170 b, and the materials of the metal gates 160 a and 160 b is different from the materials of the poly gates 170 a and 170 b, gate height loss in a centre of an array aligned by the semiconductor devices 100 caused by dishing or erosion effect during the CMP process can be reduced. For example, more than one semiconductor devices may be aligned to form an array, and due to the mechanical restrictions of the CMP process, the centre of the array is more likely to suffer stronger grinding force than the peripheral of the array. As such, gate height loss in the centre of the array may be obvious. However, the CMP process may slow down when being performed on the poly gates 170 a and 170 b of the semiconductor device 100 in the present disclosure since the materials of the poly gates 170 a and 170 b have a larger density relative to the materials of the metal gates 160 a and 160 b. Therefore, the dishing or erosion phenomenon can be reduced, thus improving the uniformity of the array and achieving a larger process window for the subsequent fabricating processes. Additionally, there's no need for enlarging the size of the array to reduce the distribution density of the metal gates 160 a and 160 b for the concern of the gate height loss. Therefore, numbers of the semiconductor device 100 in the array can be reduced, thus lowering the associated costs.

After the metal gate stacks 160 a and 160 b are formed, the semiconductor device 100 is formed. As mentioned above, the semiconductor device 100 may include a pair of LDMOS transistors, in which the two identical LDMOS transistors in the semiconductor device 100 are symmetrical with respect to a shared drain region 130, in a cross-sectional view shown in FIG. 12. In such an embodiment, the semiconductor device 100 includes a substrate 110, two first wells 120 a and 120 b, a second well 130, two source regions 140 a and 140 b, a drain region 150, two metal gates 160 a and 160 b, and two poly gates 170 a and 170 b. The first wells 120 a and 120 b and the second well 130 are within the substrate 110, in which the second well 130 is between the first wells 120 a and 120 b, and the first wells 120 a and 120 b and the second well 130 are separated by portions of the substrate 110. The source regions 140 a and 140 b are respectively within the first wells 120 a and 120 b, and the drain region 150 is within the second well 130 and between the two poly gates 170 a and 170 b to be shared by the two LDMOS transistors. The metal gate 160 a is partially over the first well 120 a and partially over the second well 130, and the metal gate 160 b is partially over the first well 120 b and partially over the second well 130. The poly gates 170 a and 170 b are over the second well 130. The metal gates 160 a and 160 b and the drain region 130 are on opposite sides of the poly gates 170 a and 170 b, and the poly gates 170 a and 170 b and the source regions 120 a and 120 b are on opposite sides of the metal gates 160 a and 160 b.

In some embodiments, the width W4 of each of the poly gates 170 a and 170 b is in a range from about 0.02 μm to about 0.2 μm, and a width W5 of each of the metal gates 160 a and 160 b is in a range from about 0.1 μm to about 2 μm. Stated differently, a width ratio of the poly gates 170 a and 170 b to the metal gates 160 a and 160 b is in a range from about 0.1 to about 0.2 μm. Such a width ratio implies a suitable area ratio of the poly gates 170 a and 170 b to the metal gates 160 a and 160 b, thus providing a suitable density for the array aligned by the semiconductor device 100. As such, the CMP process can be moderately performed on the array. If the width ratio of the poly gates 170 a and 170 b to the metal gates 160 a and 160 b is too small, the gate height loss in the centre of the array may be obvious due to the small density; if the width ratio of the poly gates 170 a and 170 b to the metal gates 160 a and 160 b is too large, the density of the semiconductor device 100 may be too large, and thus making it difficult for the CMP process to be performed.

In some embodiments, a distance D1 between the metal gates 160 a and 160 b and the poly gates 170 a and 170 b is in a range from about 0.1 μm to about 1 μm, and a distance D2 between the poly gates 170 a and 170 b is in a range from about 0.1 μm to about 1 μm. Such distances D1 and D2 provide a suitable distribution density for the poly gates 170 a and 170 b. More specifically, if the distance D1 between the metal gates 160 a and 160 b and the poly gates 170 a and 170 b and the distance D2 between the poly gates 170 a and 170 b are respectively smaller than 0.1 μm, the distribution density of the poly gates 170 a and 170 b may be too large, thus making it difficult for the CMP process to be performed; if the distance D1 between the metal gates 160 a and 160 b and the poly gates 170 a and 170 b and the distance D2 between the poly gates 170 a and 170 b are larger than 1 μm, the distribution density of the poly gates 170 a and 170 b may be too small, such that the gate height loss in the centre of the array aligned by the semiconductor device 100 may be obvious.

FIG. 13 is a cross-sectional view illustrating a semiconductor device 100 a in accordance with another embodiment of the present disclosure, in which the semiconductor device 100 a is fabricated by the alternative embodiment mentioned above in FIG. 10. In this embodiment, the gate dielectric layers 194 a and 194 b includes a high-k dielectric material and would not have been removed when the gate electrode layers 196 a and 196 b and the hard masks 198 a and 198 b were removed to form a shallower trench, thus the high-k gate dielectric layers 164 a and 164 b (see FIG. 12) need not be formed in the alternative embodiment. After the removal of the gate electrode layers 196 a and 196 b and the hard masks 198 a and 198 b, the first conductive layers 166 a and 166 b and the second conductive layers 168 a and 168 b are formed in the trench, and thereafter the materials outside the trench are removed in a CMP process. As such, the metal gate stack 162 a is formed by the first conductive layer 166 a, the second conductive layer 168 a, and the gate dielectric layer 194 a, and the metal gate stack 162 b is formed by the first conductive layer 166 b, the second conductive layer 168 b, and the gate dielectric layer 194 b. That is, the metal gate stacks 162 a and 162 b includes the gate dielectric layers 194 a and 194 b, the first conductive layers 166 a and 166 b, and the second conductive layers 168 a and 168 b in this embodiment.

Based on the aforementioned descriptions, since the semiconductor device includes the metal gates and the poly gates with different materials, gate height loss in a centre of an array aligned by the semiconductor devices caused by dishing or erosion effect during the CMP process can be reduced, thus improving the uniformity of the array and achieving a larger process window for the subsequent fabricating processes. Accordingly, there's no need for enlarging the size of the array to reduce the distribution density of the metal gates for the concern of the gate height loss. Therefore, numbers of the semiconductor device in the array can be reduced, thus lowering the associated costs.

In some embodiments, a semiconductor device includes a substrate, a first well, a second well, a metal gate, a poly gate, a source region, and a drain region. The first well and the second well are within the substrate. The metal gate is partially over the first well. The poly gate is over the second well. The poly gate is separated from the metal gate, and a width ratio of the poly gate to the metal gate is in a range from about 0.1 to about 0.2. The source region and the drain region are respectively within the first well and the second well.

In some embodiments, a semiconductor device includes a substrate, two first wells, a second well, a first metal gate, a second metal gate, a first poly gate, a second poly gate, a source region, and a drain region. The first wells and the second well are within the substrate, in which the second well is between the first wells. The first metal gate is partially over one of the two first wells. The second metal gate is partially over the other of the two first wells. The first poly gate and the second poly gate are over the second well, in which the first poly gate is separated from the second poly gate. The source region and the drain region are respectively within the first well and the second well.

In some embodiments, a method of fabricating a semiconductor device includes: forming a first well and a second well in a substrate, in which the first well and the second well are doped with different types of dopants; forming a dummy gate partially over the first well and partially over the second well, in which the dummy gate includes a dummy gate stack; forming a poly gate over the second well, in which the poly gate includes a poly gate stack, and a gap is between the dummy gate and the poly gate; forming a source region and a drain region respectively in the first well and the second well, in which the source region and the drain region are doped with a same type of dopants; and replacing the dummy gate stack with a metal gate stack.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method of fabricating a semiconductor device, comprising: forming a first well and a second well in a substrate, wherein the first well and the second well are doped with different types of dopants; forming a dummy gate partially over the first well and partially over the second well, wherein the dummy gate comprises a dummy gate stack; forming a poly gate over the second well, wherein the poly gate comprises a poly gate stack, and a gap is between the dummy gate and the poly gate; forming a source region and a drain region respectively in the first well and the second well, wherein the source region and the drain region are doped with a same type of dopants; and replacing the dummy gate stack with a metal gate stack, comprising: disposing a hard mask covering the poly gate; removing the dummy gate stack to form a trench; filling the trench with materials of the metal gate stack while the hard mask remains covering the poly gate; performing a first planarization process to the materials of the metal gate stack, such that the hard mask is exposed; removing the hard mask after performing the first planarization process; and performing a second planarization process after removing the hard mask, such that a top surface of the metal gate stack is coplanar with a top surface of the poly gate.
 2. The method of claim 1, wherein forming the dummy gate and the poly gate comprises: forming the dummy gate stack and the poly gate stack; and forming first spacers and second spacers respectively on sidewalls of the dummy gate stack and sidewalls of the poly gate stack.
 3. The method of claim 1, wherein the materials of the metal gate stack further covers the hard mask.
 4. The method of claim 1, wherein a material of the hard mask comprises polymer.
 5. The method of claim 1, wherein removing the hard mask is performed by an ashing process.
 6. A method of fabricating a semiconductor device, comprising: forming a first isolation structure and a second isolation structure in a substrate, wherein a portion of the substrate extends from the first isolation structure to the second isolation structure; forming a first dummy gate structure and a second dummy gate structure over the portion of the substrate; forming a source region and a drain region in the portion of the substrate, wherein the source region is between the first isolation structure and the first dummy gate structure, and the drain region is between the second dummy gate structure and the second isolation structure; forming a resist protection layer over the first and second dummy gate structures, wherein the resist protection layer extends from the first dummy gate structure to the second dummy gate structure; after forming the resist protection layer, forming a hard mask over the second dummy gate structure, wherein the hard mask covers the second dummy gate structure but exposes the first dummy gate structure, and the hard mask is in contact with the resist protection layer; and replacing the first dummy gate structure with a metal gate structure.
 7. The method of claim 6, wherein forming the resist protection layer is such that the resist protection layer is spaced apart from the source region and the drain region.
 8. The method of claim 6, wherein a width of the first dummy gate structure is greater than a width of the second dummy gate structure.
 9. The method of claim 6, wherein both the first and second dummy gate structures are closer to the first isolation structure than to the second isolation structure.
 10. The method of claim 6, wherein both the first and second dummy gate structures are between the source region and the drain region.
 11. The method of claim 6, wherein replacing the first dummy gate structure with the metal gate structure is performed after forming the hard mask.
 12. The method of claim 6, wherein replacing the first dummy gate structure with the metal gate structure is performed while the hard mask remains covering the second dummy gate structure.
 13. The method of claim 6, further comprising removing the hard mask after replacing the first dummy gate structure with the metal gate structure.
 14. The method of claim 6, wherein the hard mask partially covers the resist protection layer.
 15. The method of claim 6, wherein a material of the hard mask comprises metal.
 16. A method of fabricating a semiconductor device, comprising: forming a first dummy gate structure, a second dummy gate structure, a third dummy gate structure, and a fourth dummy gate structure over a substrate, wherein the second and third dummy gate structures are between the first and fourth dummy gate structures; forming a first source region, a second source region, and a drain region in the substrate such that the first and second dummy gate structures are between the first source region and the drain region and the third and fourth dummy gate structures are between the second source region and the drain region; forming a resist protection layer between the first and second dummy gate structures; forming a hard mask to cover the drain region, the second dummy gate structure, and the third dummy gate structure but not the first and fourth dummy gate structures; and after forming the hard mask, replacing the first and fourth dummy gate structures with metal gate structures.
 17. The method of claim 16, wherein forming the hard mask is such that the hard mask is in contact with the resist protection layer.
 18. The method of claim 16, further comprising forming a first isolation structure and a second isolation structure in the substrate and adjacent to each other, wherein the first, second, third, and fourth dummy gate structures are all between the first and second isolation structures.
 19. The method of claim 16, further comprising forming a first well region and a second well region in the substrate and spaced apart from each other, wherein a first portion of the first dummy gate structure is directly over the first well region and a second portion of the first dummy gate structure is directly over the second well region.
 20. The method of claim 19, wherein the second and third dummy gate structures are both directly over the second well region and not directly over the first well region. 